Fix Calvin determinism gate failure in test-only sub-plan task builder

[Copilot]

The Static gates / Static gates (pull_request) job failed in check_calvin_determinism.sh due to an unmarked Instant::now() use in the Calvin write-path scan scope (sub_plan.rs). The call is test-only and not part of WAL/state mutation, but it still must satisfy the gate’s line-local marker rule.

• Root cause

  • nodedb/src/data/executor/handlers/transaction/sub_plan.rs had Instant::now() inside #[cfg(test)] build_dummy_task.
  • Existing rationale text was not on the same line or directly preceding line, so the grep-based determinism gate flagged it.

• Code change

  • Added/moved an inline // no-determinism: ... marker to the Instant::now() site (or immediately above it) so the call is explicitly exempt per gate contract.
  • Kept behavior unchanged; this is a gate-compliance annotation fix for a test-only helper.

• Gate-contract example

  // no-determinism: test-only ephemeral deadline, not persisted to Calvin/WAL state.
  std::time::Instant::now() + std::time::Duration::from_secs(60)